1. Field of the Invention
The instant invention relates to an electrophysiology apparatus used to measure electrical activity occurring in a heart of a patient and to visualize the electrical activity and/or information related to the electrical activity. In particular, the instant invention relates to three-dimensional mapping of the electrical activity and/or the information related to the electrical activity.
2. Background Art
The heart contains two specialized types of cardiac muscle cells. The majority, around ninety-nine percent, of the cardiac muscle cells is contractile cells, which are responsible for the mechanical work of pumping the heart. Autorhythmic cells comprise the second type of cardiac muscle cells, which function as part of the autonomic nervous system to initiate and conduct action potentials responsible for the contraction of the contractile cells. The cardiac muscle displays a pacemaker activity, in which membranes of cardiac muscle cells slowly depolarize between action potentials until a threshold is reached, at which time the membranes fire or produce an action potential. This contrasts with a nerve or skeletal muscle cell, which displays a membrane that remains at a constant resting potential unless stimulated. The action potentials, generated by the autorhythmic cardiac muscle cells spread throughout the heart triggering rhythmic beating without any nervous stimulation.
The specialized autorhythmic cells of cardiac muscle comprising the conduction system serve two main functions. First, they generate periodic impulses that cause rhythmical contraction of the heart muscle. Second, they conduct the periodic impulses rapidly throughout the heart. When this system works properly, the atria contract about one sixth of a second ahead of ventricular contraction. This allows extra filling of the ventricles before they pump the blood through the lungs and vasculature. The system also allows all portions of the ventricles to contract almost simultaneously. This is essential for effective pressure generation in the ventricular chambers. The rates at which these autorhythmical cells generate action potentials differ due to differences in their rates of slow depolarization to threshold in order to assure the rhythmical beating of the heart.
Normal autorhythmic cardiac function may be altered by neural activation. The medulla, located in the brainstem above the spinal cord, receives sensory input from different systemic and central receptors (e.g., baroreceptors and chemoreceptors) as well as signals from other brain regions (e.g., the hypothalamus). Autonomic outflow from the brainstem is divided principally into sympathetic and parasympathetic (vagal) branches. Efferent fibers of these autonomic nerves travel to the heart and blood vessels where they modulate the activity of these target organs. The heart is innervated by sympathetic and vagal fibers. Sympathetic efferent nerves are present throughout the atria (especially in the sinoatrial node) and ventricles, including the conduction system of the heart. The right vagus nerve primarily innervates the sinoatrial node, whereas the left vagus innervates the atrial-ventricular node; however, there can be significant overlap in the anatomical distribution. Efferent vagal nerves also innervate atrial muscle. However, efferent vagal nerves only sparsely innervate the ventricular myocardium. Sympathetic stimulation increases heart rate and conduction velocity, whereas parasympathetic (vagal) stimulation of the heart has opposite effects.
An arrhythmia occurs when the cardiac rhythm becomes irregular, i.e., too fast (tachycardia) or too slow (bradycardia), or the frequency of the atrial and ventricular beats are different. Arrhythmias can develop from either altered impulse formation or altered impulse conduction. The former concerns changes in rhythm that are caused by changes in the pacemaker cells resulting in irregularity or by abnormal generation of action potentials by sites other than the sinoatrial node, i.e., ectopic foci. Altered impulse conduction is usually associated with complete or partial blockage of electrical conduction within the heart. Altered impulse conduction commonly results in reentry, which can lead to tachyarrhythmias. Reentry can take place within a small local region or it can occur, for example, between the atria and ventricles (global reentry). Reentry requires the presence of a unidirectional block within a conducting pathway usually caused by partial depolarization of the pacemaker cells. Arrhythmias can be either benign or more serious in nature depending on the hemodynamic consequences of arrhythmias and their potential for changing into lethal arrhythmias.
Electrophysiology studies are used to identify and treat these arrhythmias. In one exemplary system, a measurement system introduces a modulated electric field into the heart chamber. The blood volume and the moving heart wall surface modify the applied electric field. Electrode sites within the heart chamber passively monitor the modifications to the field and a dynamic representation of the location of the interior wall of the heart is developed for display to the physician. Electrophysiology signals generated by the heart itself are also measured at electrode sites within the heart and these signals are low pass filtered and displayed along with the dynamic wall representation. This composite dynamic electrophysiology map may be displayed and used to diagnose the underlying arrhythmia.
In addition to mapping for diagnosis, the measurement system can also be used to physically locate a therapy catheter in a heart chamber. A modulated electrical field delivered to an electrode on this therapy catheter can be used to show the location of the therapy catheter within the heart. The therapy catheter location can be displayed on the dynamic electrophysiology map in real time along with the other diagnostic information. Thus the therapy catheter location can be displayed along with the intrinsic or provoked electrical activity of the heart to show the relative position of the therapy catheter tip to the electrical activity originating within the heart itself. Consequently, the physician can guide the therapy catheter to any desired location within the heart with reference to the dynamic electrophysiology map.
The dynamic electrophysiology map is generally produced in a step-wise process. First, the interior shape of the heart is determined. This information is derived from a sequence of geometric measurements related to the modulation of the applied electric field. Knowledge of the dynamic shape of the heart is used to generate a representation of the interior surface of the heart. Next, the intrinsic electrical activity of the heart is measured. The signals of physiologic origin are passively detected and processed such that the magnitude of the potentials on the wall surface may be displayed on the wall surface representation. The measured electrical activity is displayed on the wall surface representation in any of a variety of formats, for example, in various colors or shades of a color. Finally, a location current may be delivered to a therapy catheter within the same chamber. The potential sensed from this current may be processed to determine the relative or absolute location of the therapy catheter within the chamber. These various processes occur sequentially or simultaneously several hundred times a second to give a continuous image of heart activity and the location of the therapy device.
One exemplary system for determining the position or location of a catheter in the heart is described in U.S. Pat. Nos. 5,697,377 (the '377 patent) and 5,983,126 (the '126 patent) to Wittkampf. The '377 patent and the '126 patent are hereby incorporated herein by reference in their entirety. In the Wittkampf system, current pulses are applied to orthogonally placed patch electrodes placed on the surface of the patient. These surface electrodes are used to create axis specific electric fields within the patient. The Wittkampf references teach the delivery of small amplitude, low current pulses supplied continuously at three different frequencies, one on each axis. Any measurement electrode placed in these electric fields (for example within the heart) measures a voltage that varies depending on the location of the measurement electrode between the various surface electrodes on each axis. The voltage across the measurement electrode in the electric field in reference to a stable positional reference electrode indicates the position of the measurement electrode in the heart with respect to that reference. Measurement of the difference in voltage over the three separate axes gives rise to positional information for the measurement electrode in three dimensions.